1. Field of the Invention
The present invention relates to data acquisition or reception and, in particular, to data acquisition in a spread-spectrum, time-division multiplexed multi-line wireless telephone system.
2. Description of the Related Art
Digital data transmission from a transmitter to a receiver requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter and successfully recovered or acquired by the receiver. In digital wireless telephone systems, for example, a wireless (cordless) telephone handset unit communicates via digital radio signals with a base unit, which is typically connected via a standard telephone line to an external telephone network. Each handset and the base comprise a transceiver, having a transmitter and receiver. In such a system, a user may employ the wireless handset to engage in a telephone call with another user through the base unit and the telephone network.
Multi-line wireless telephone systems are in use in various situations, such as businesses with many telephone users. Such systems employ a base unit that communicates with up to N handsets in real time, typically with digital communications schemes, such as a spread-spectrum, time division multiplex (TDM) schemes such as time division multiple access (TDMA). In a spread spectrum system, bandwidth resources are traded for performance gains, in accordance with the so-called Shannon theory. The advantages of a spread-spectrum system include low power spectral density, improved narrowband interference rejection, built-in selective addressing capability (with code selection), and inherent channel multiple access capability. Spread-spectrum systems employ a variety of techniques, including direct sequencing (DS), frequency hopping (FH), chirp systems, and hybrid DS/FH systems.
In a TDMA system, a single RF channel is used, and each handset transmits and receives audio data packets as well as non-audio data packets during dedicated time slices or time slots within an overall TDMA cycle or epoch. Other communications schemes include frequency division multiple access (FDMA), code division multiple access (CDMA), and combinations of such schemes. Various modulation schemes are employed, such as carrierless amplitude/phase (CAP) and quadrature amplitude modulation (QAM).
Digital data is typically transmitted as modulated signals over a transmission medium, such as the RF channel, in the form of binary bits of data. (Other transmission media often used for digital communications include asymmetric digital subscriber loop (ADSL) systems or cable modem systems.) The digital data is often modulated and transmitted in complex digital data form, in which the transmitted data comprises symbols from which the original data can be reconstructed by the receiver. Complex digital symbol data typically comprises real (in-phase, or xe2x80x9cIxe2x80x9d) data, and imaginary (quadrature, or xe2x80x9cQxe2x80x9d) data (I, Q pairs). Each symbol of an I,Q pair may be a multi-bit number, and represent a location of a constellation, mapped against a quadrant. Each symbol is mapped or assigned to a prescribed coordinate in a four-quadrant grid-like constellation using a look-up table (e.g., a ROM). A prescribed number of symbols occupy assigned areas in each quadrant, depending on the encoding scheme. Depending on the number of bits/symbol of a given encoding scheme, each quadrant of the constellation contains a number of symbols at prescribed coordinates with respect to quadrature I and Q axes. For example, in the QPSK encoding scheme, each sample has one of four phase positions, one for each quadrant, so that each symbol pair represents two bits of data.
To transmit a given input data value in a complex data system, the input data value to be transmitted is mapped to a symbol pair or pair of coordinates I,Q of a corresponding constellation point on a complex signal constellation having real and imaginary axes I and Q. These I,Q symbols, which represent the original data value, are then transmitted as part of data packets by a modulated channel. A receiver can recover the I,Q pairs and determine the constellation location therefrom, and perform a reverse-mapping to provide the original input data value or a close approximation thereof.
In a spread spectrum system, each symbol is transmitted by a string of xe2x80x9csub-symbolsxe2x80x9d or xe2x80x9cchipsxe2x80x9d, which is typically derived by multiplying the symbol (which may be either a 1 or xe2x88x921, in some schemes) times a pseudo-random number (PN) binary string of a certain length (number of chips C). Such systems are thus characterized by a chip rate, which is related to the symbol rate. Spread spectrum systems may also be used, in general, to transmit any digital data, whether in complex format or not, and whether or not in a TDMA system.
In a spread spectrum system, a signal represents successive symbols, by means of successive chips of symbols. A received signal is sampled to provide samples. Samples thus represent a signal, which itself represents chips, which represent symbols. The receiver side of a transceiver samples a received signal with an ADC, which provides samples representative of the signal, which in turn represents symbols. The transmitter side of a transceiver converts symbols into analog samples that constitute a signal, with a digital-to-analog converter (DAC).
As noted above, digital data transmission requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter (e.g., the transmitter of the base unit transceiver) and successfully recovered by the receiver (e.g., the receiver of a given handset transceiver). For example, the receiver side of a data transmission in a spread-spectrum digital wireless telephone systems employs a variety of functions to recover data from a transmitted RF signal. These functions can include: timing recovery for symbol synchronization, carrier recovery (frequency demodulation), and gain. The receiver thus includes, inter alia, an automatic gain control (AGC) loop, carrier tracking loop (CTL), and timing loop for each link.
Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. In some receivers, the received signal is sampled at a multitude of the transmitter symbol rate. For example, some receivers sample the received signal at twice the transmitter symbol rate. In any event, the sampling clock of the receiver must be synchronized to the symbol clock of the transmitter. Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. AGC tracks signal strength and adjusts the gain, for example to help compensate for the effects of transmission channel disturbances upon the received signal. AGC, along with other equalization techniques, can help remove intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. AGC is important, therefore, because multiple handsets and/or base stations in close proximity can interfere with one another, and thus the system transceivers must use minimal needed gain to avoid system saturation leading to interference, and also to more efficiently utilize battery power. These and related functions, and related modulation schemes and systems, are discussed in greater detail in Edward A. Lee and David G. Messerschmitt, Digital Communication, 2d ed. (Boston: Kluwer Academic Publishers, 1994).
Receivers require a relatively stable source of a sampling clock signal, which is also controllable so that it can be locked to the transmitter symbol clock. Voltage controlled crystal oscillators (VCXO""s) have been used for this function, since the clock signal produced by a VCXO is stable but controllable over a relatively narrow range, to allow it to be locked to the transmitter symbol clock. Other types of timing recovery systems may also be employed, such as the xe2x80x9cTiming Recovery System for a Digital Signal Processorxe2x80x9d described in European Patent Application No. EP 0 793 363, European filing date Feb. 20, 1997, Applicant Thomson Consumer Electronics, Inc., inventors Knutson, Ramaswamy, and McNeely (Knutson et al.).
In a spread spectrum multi-line wireless telephone system, as in all spread spectrum systems, it is important for each transceiver in the system to be able to accurately receive transmitted signals and, in particular, to sample at the appropriate frequency and phase so as to improve signal reception and recovery. In a spread spectrum TDMA system, it is also important for each transceiver to be able to detect a valid data signal and also to detect the guard band indicating the end of the data packet transmission.
A receiver of a handset transceiver, in a wireless telephone system having a plurality of wireless handset and a base unit having a base transceiver. Each handset has a handset transceiver for establishing a time-division multiple access (TDMA) link over a shared channel with the base unit via the base transceiver, in which each handset communicates during an exclusive time slot of a TDMA scheme that allocates time slots to handsets during an exclusive time slot of a TDMA scheme that allocates time slots to active handsets for receiving a spread spectrum signal comprising successive chips representing successive symbols. Each receiver has one or more demodulation loops for demodulating the received spread spectrum signal, wherein each demodulation loop is characterized by one or more demodulation loop parameters. A parallel correlator of the receiver detects a peak bin and provides the peak bin and two adjacent bins. An error estimator of the receiver adjusts the demodulation parameters in accordance with the peak bin and the two adjacent bins to optimize demodulation of the spread spectrum signal